Maha Shivratri (Devanagari:महाशिवरात्रि, Kannada:ಮಹಾ ಶಿವರಾತ್ರಿ, Tamil:மகா சிவராத்திரி, Telugu:మహాశివరాత్రి), is a Hindu festival celebrated every year in reverence of Lord Shiva. Other alternate common names/spellings include Maha Sivaratri, Shivaratri, Sivarathri, Shivaratra. Shivaratri literally means the great night of Shiva or the night of Shiva. It is celebrated every year on the 13th night/14th day of the Maagha or Phalguna month of Hindu calendar.Since many different calendars are followed by various ethno-linguistic groups of Nepal and India, the month and the Tithi name are not uniform all over
Nepal and India.
Celebrated in the dark fortnight or Krishna Paksha (waning moon) of the month of Maagh according to the Shalivahana or Gujarati Vikrama or Phalguna according to the Vikrama era. The festival is principally celebrated by offerings of Bael or Bilva/Vilvam leaves to Lord Shiva, all day fasting and an all night long vigil. In accordance with scriptural and discipleship traditions, penances are performed in order to gain boons in the practice of Yoga and meditation, in order to reach life’s summum bonum steadily and swiftly.

Pashupatinath Temple in Kathmandu has its significance and can observe the real tradition of Worshiping Lord Shiva. During this period, thousands of Sadhu’s are pouring from different part of India and Nepal to the temple and practice their faith full night with campfire and smoking Shiva’s Prasada (traditionally it is Marijuana).
A week long International Mandi Shivratri Fair held at Mandi in the Indian state of Himachal Pradesh every year is one of the major tourist attractions in the state.
Legends
According to a legend, Parvati performed tapas, prayed and meditated on this day to ward off any evil that might befall her husband on the moonless night. Since then, Maha shivaratri is believed to be an auspicious occasion for women to pray for the well-being of their husbands and sons. An unmarried woman prays for a husband like Shiva, who is considered to be the ideal husband.Another legend of the day is that Shiva and Shakti married each other.
There is one more legend of shivratri associated with Samudra Manthan, a battle between devas and asuras during which a pot of poison came out of the ocean and on the request of gods, Lord Shiva drank the poison. The poison was so potent that it changed the color of His neck to blue. For this reason, Lord Shiva is also called Neelkanth.
Rituals of Maha Shivratri
From the very early morning, Shiva temples are flocked by devotees, mostly women, who come to perform the traditional Shivalinga worship and hence hope for favours from the god. Devotees bathe at sunrise, preferably in the Ganga, or any otherholy water source (like the Shiva Sagartank at Khajurao).
This is a purification rite, an important part of all Hindu festivals. Wearing a clean piece of clothing after the holy bath, worshipers carry pots of water to the temple to bathe the Shivalinga. They offer prayers to the sun, Vishnu and Shiva.
Women pray for the well-being of their husbands and sons. An unmarried woman prays for a husband like Shiva, who is considered to be the ideal husband. The temple reverberates with the sound of bells and shouts of “Shankerji ki Jai” or ‘Hail Shiva’. Devotees circumambulate the linga, three or seven times, and then pour water over it. Some also pour milk.
According to the Shiva Purana, the Mahashivaratri worship must incorporate six items:
Bathing the Shiv Linga with water, milk and honey, and Wood apple or bel leaves added to it, representing purification of the soul;
The vermilion paste applied on the Shiv linga after bathing it, representing virtue;
Offering of fruits, which is conducive to longevity and gratification of desires;
Burning incense, yielding wealth;
The lighting of the lamp which is conducive to the attainment of knowledge;
And betel leaves marking satisfaction with worldly pleasures.
Tripundra refers to the three horizontal stripes of holy ash applied to the forehead by worshippers of Lord Shiva. These stripes symbolise spiritual knowledge, purity and penance (spiritual practice of Yoga), so also they represent the three eyes of Lord Shiva.
Wearing a rosary made from the rudraksha seed of the rudraksha tree (said to have sprung from the tears of Lord Shiva) when worshipping Lord Shiva is ideal. A rudraksha seed is a mahogany-like color, sometimes black, and sometimes may have traces of sacred sandalwood powder, turmeric, kumkum, or holy ash if the rosary was used in worship ceremonies or anointed.
Source: Wikipedia